Conventionally, a technique has been developed for displaying images taken by, for example, a camera or a sequence of photographs, such as a motion picture, based on external data in real time directly in the neighborhood of eyeballs of a user or directly on a retina thereof.
For example, Japanese Published Unexamined Patent Application No. 2006-85011 or Japanese Published Unexamined Patent Application No. 2006-98827 can be mentioned as an example of a display device that adopts holography. Additionally, a retina scanning display device that displays images directly on a retina of a user can also be used as another transmissive display device.
As an example in which these display devices are adapted, it is conceivable to use these devices for low-vision or visually-impaired persons. In detail, the scenery around (especially, in front of) a visually-impaired person who is a user is imaged by a camera, and is displayed before the eyes of the user by a display device so as to be used as visual assistance. Generally, in a low-vision or visually-impaired person who has residual vision, it is bad to give great light energy to the retina of the person, and therefore external light is prevented from entering the eyes of the person by wearing lenses having a light blocking effect (hereinafter, referred to as “sunglass lenses”).
For example, the structure of FIG. 9 or that of FIG. 10 is conceived as a display unit for visual assistance in which sunglass lenses are used together.
FIG. 9 is a view showing an example of a display unit using a transmissive display device that adopts holography. A sunglass lens 101 is disposed on the outermost side (i.e., on the object side) of this display unit, and a transmissive display device 102 is disposed inside the sunglass lens 101. A corrective lens 105 is disposed inside the transmissive display device 102 (i.e., disposed closer to the eyeball). The transmissive display device 102 includes a transparent board and an image projecting part 104 disposed on the upper part of the board. In the transmissive display device 102, both a real image that has passed through the transparent board 103 and a virtual image of a hologram can be viewed in an overlapped state, and therefore this device is called “transmissive.”
FIG. 10 is a view showing an example of a display unit using a non-transmissive display device. A sunglass lens 106 is disposed on the outermost side of this display unit, and a monitor 107 of a non-transmissive display device is disposed inside the sunglass lens 106. Unlike the transmissive display device, the non-transmissive display device allows a real image to be viewed only around the monitor 107.
However, in the display unit of FIG. 9, three lenses are overlapped together in the front-back direction, thus leading to a complex structure and an inferior outward appearance. Therefore, it is preferable to unite the sunglass lens 101 and the lens 105 together and dispose the lens 108 having a sunglass function on the eyeball side of the transmissive display device 102 in the same way as in the display unit of FIG. 11.
Generally, the non-transmissive display device of the display unit of FIG. 10 has a function to allow the device itself to adjust visibility, and therefore another lens is not required. However, if the monitor 107 is thick, problems will be caused in the fact that its thickness makes it difficult to intercept external light and in the fact that the lenses-wearing person has an uncomfortable feeling because the sunglass lens 106 used as a component is kept greatly away from the eyes forwardly. Therefore, it is preferable to dispose the sunglass lens 106 closer to the eyeball than the monitor 107 as shown in FIG. 12.
However, there are many cases in which the characteristic of blocking light on the short wavelength side having great energy, more specifically, the characteristic of blocking light on the ultraviolet side from near blue is normally given to the sunglass lenses 101 and 106 as shown in a transmission characteristic graph of FIG. 13. In other words, visible light that is external light is viewed as yellowish light while allowing light excluding blue light to remain. However, a color image is reproduced by combining light's three primary colors R, G, and B together, and therefore, if a structure in which the sunglass lenses are disposed closer to the eyeball than the display device is adopted as in FIG. 11 or FIG. 12, B light will likewise be almost cut from light of images displayed on the display device as shown in the graph of FIG. 13, and, as a result, the color image loses blue, and becomes yellowish as a whole. On the other hand, in the sunglass lenses, light on the blue side is originally cut, and therefore there is no difference in color (i.e., hue) after all, and the scenery becomes yellowish as a whole before the eyes of the user, and, as a result, the real image and the virtual image must be distinguished from each other only by the difference in brightness. Therefore, it becomes difficult to draw a distinction between the real image and the virtual image.
On the other hand, in an example in which the sunglass lenses 101 and 106 are disposed on the more external side than the display device (i.e., on the object side) as in FIG. 9 and FIG. 10, when the scenery is viewed at nighttime, there is a case in which it is difficult to draw a distinction between a virtual image of a hologram and a real image that has passed through the sunglass lenses 101 and 106 and that has been overlapped with the virtual image, because the scenery is dark, and saturation is low.
The foregoing description is given as a noticeable problem occurring in a visually-impaired person when the display unit is structured such that the sunglass lenses are used in combination with the display device. However, even if the person is not a visually-impaired person, the same problem will arise in the display unit in which the sunglass lenses are used in combination with the display device.
Therefore, a technique capable of easily drawing a distinction between a real image and a photographic image has been required regardless of a positional relationship between sunglass lenses and a display device.
The present invention has been made in consideration of the problems existing in the conventional technique, and it is an object of the present invention to provide a display unit that enables a user to easily distinguish a real image and a photographic image from each other regardless of a positional relationship between sunglass lenses and a display device.